A. Field of the Invention
The present invention relates generally to ionization chambers for detecting radiation emitted by radioactive material, and more particularly to an ionization chamber having a drift region and a detection region. The chamber is arranged to contain a suitable gas so that upon exposure to radiation, ionized electrons are produced which drift at at relatively low speed toward the detection region, and are thereafter accelerated and detected as in a proportional counter.
B. Discussion of the Prior Art
Various ionization chambers are known in the art, such chambers being used to detect the presence of fast moving particles associated with x-rays, gamma rays, neutrons, alpha particles, and the like. These chambers have many uses, such as in research and in nuclear instrumentation wherein the level of radiation intensity at a given point is to be measured.
Naturally, gas filled ionization chambers operate on the principle that when ionizing particles, which carry an electric charge, pass near the gas atoms they cause electrons of the atoms to be removed thereby ionizing the atoms. A static electric field is provided through the chambers so that the freed electrons are caused to drift along the direction of the field, and wire or plate electrodes located in the chamber then detect the presence of these electrons as they pass near or impinge on the electrodes. A resistor is connected to one of the electrodes, usually the anode, and a detection current induces a voltage drop across this resistor. The voltage drop is amplified to provide a pulse waveform which can be viewed on an oscilloscope, or otherwise processed by various waveform analyzing instruments. By analyzing the voltage waveforms produced, it is possible to determine the number of ionization electrons impinging on or passing near the detecting electrode over a given period of time. From this information, it is possible to determine physical properties of the particles within the chamber.
In the event an ionization chamber is used to detect ionization electrons produced by a single particle, it will be appreciated that the electrode current produced only by those ionization electrons freed by the particle will be quite small. Accordingly, an amplifier must be used to increase the voltage produced by this current across the electrode resistor. A larger pulse may be obtained if the ionization chamber is arranged to operate as a proportional counter. In such a case, the electric field is increased so that the ionization electrons initially freed by the particle are themselves accelerated sufficiently to cause further ionization as they pass near other gas atoms. As the field strength is increased up to a predetermined value, the output pulse waveform will be directly proportional to the original ionization of the gas as caused by the particle alone. Thus, the proportional counter uses gas multiplication as a linear amplifying device.
Problems have arisen, however, in the use of proportional counters for determining physical properties of relativistic particles. For example, it has become common knowledge that with ionization chambers using multi-wire electrodes, the output pulse waveform changes as a function of the distance between the particle trajectory and the anode wire. The most pronounced changes are observed (for a particle trajectory perpendicular to the wire plane) when the particle passes at or very near the anode wire. The pulse rise time becomes slower since the ionization distributed along the trajectory arrives at the anode in a sequence corresponding to the drift time. If the chamber is not operated as a proportional counter, it was found that the particle position resolution for trajectories close to the anode is unsatisfactory. This effect has been explained by the stochastic nature of the ionization which defines a number of ionization clusters along the trajectory. If suitable instrumentation is used, the anode current is observed as showing a structure with several maxima which may be explained by the statistical fluctuation in the primary ionization.
It has been realized that counting the number of clusters would greatly improve high energy particle identification based on the relativistic rise of energy loss. This is because most of the relativistic rise of energy loss observed in an ionization chamber is primarily due to the increase of the number of collisions, and depends only slightly upon the change of energy loss in a single collision which corresponds to the amount of ionization in one cluster. This is responsible for large fluctuations in the energy loss measurement which therefore makes it difficult to measure the relativistic rise. Furthermore, the signal provided by the anode current for single clusters is not well resolved. Each peak of the signal still consists of several clusters, since the response of the counter to a single cluster is too slow (due to low drift velocity of positive ions) compared to the mean drift distance of the clusters in an ordinary proportional counter.
A proportional counter radiation camera is known, this camera being disclosed in U.S. Pat. No. 3,786,270 as having multi-wire electrodes arranged in a detecting region which is located next to a series of parallel spaced field plates or electrodes which provide a linear drift field. The purpose of the drift field is to provide a greater active gas volume and thereby increase the detection efficiency of the camera, according to the patent. Further, the electrodes are arranged to provide output pulses having amplitudes proportional to the position of a detected event across each of the electrodes so that an image of radiation which is directed perpendicularly to the plane of the electrodes can be recorded by appropriate two-dimensional recording means associated with the camera. There is no disclosure or suggestion in U.S. Pat. No. 3,786,270 of providing a drift field region wherein relativistic particles can enter along trajectories which extend in other than a perpendicular direction relative to the electrodes, and that signals can be provided by the electrodes which serve to identify a number of properties of the particles including their identity.